In the production of refrigeration units which are commonly used in refrigerators, freezers, air conditioners, walk-in coolers, and the like, it is necessary to charge each unit with a predetermined weight of liquid refrigerant such as the refrigerants commonly referred to in the industry as R-12, R-22, R-500 and R-502 and which may be purchased under the trademark Freon. The weight of the charge of liquid refrigerant depends on the size of the refrigeration unit and commonly ranges from a few ounces, such as used in a refrigeration unit for a water cooler drinking fountain, to over 15 pounds such as used in an industrial air conditioning system. Usually, the charge is expressed in pounds and ounces avoirdupois, but there is a growing demand for charging in the metric system of grams and kilograms.
Since the density of the liquid refrigerant decreases as the temperature of the refrigerant increases, and visa versa, it has been found highly desirable to maintain the liquid refrigerant at a substantially constant temperature during the successive charging operations, for example, as disclosed in U.S. Pat. No. 2,631,437 which issued to assignee of the present invention, or to adjust the weight of the liquid refrigerant dispensed into each refrigerating unit in accordance with changes in temperature of the refrigerant. U.S. Pat. No. 2,775,870, which also issued to the assignee of the present invention, discloses a liquid refrigerant metering and dispensing device which automatically compensates for changes in temperature of the liquid refrigerant which is normally stored at room temperature or within the range of 70.degree. F. to 90.degree. F.
It has been found desirable for a liquid refrigerant dispensing or metering system to be designed and constructed for operating within an ambient temperature range of 50.degree. F. to 120.degree. F. and to compensate automatically for changes in density of the refrigerant within this temperature range. The metering and dispensing device disclosed in above mentioned U.S. Pat. No. 2,775,870 provides this automatic compensation by means of a printed electrical circuit contact board or panel which included a series of metal contact lines having a non-corroding outer layer of gold. A sliding electrical contact element is connected for axial movement with the displacement ram or cylinder and successively engages the contact lines until it contacts the line which is connected to control the solenoid valve through which hydraulic fluid enters the cylinder to actuate the displacement piston. As illustrated in the patent, the contact lines are arranged in diverging relation, and the contact board or panel is adjusted laterally relative to the axis of the piston in response to changes in temperature of the liquid refrigerant so that the stroke of the displacement ram or piston will change when the temperature of the refrigerant changes. The arrangement of the contact lines and the lateral adjustment of the contact board are calibrated according to the changes in density of a particular refrigerant with changes in temperature.
While the adjustable contact board or panel has performed very successfully for many years, contact boards have become more and more costly to produce, and a larger inventory of contact boards must be maintained to accommodate the increasing number of different liquid refrigerants and the different sizes of the metering and dispensing devices. In addition, once a contact board or panel has been designed and constructed, it is inflexible and cannot be easily corrected or altered, for example, to compensate for an error or to adjust the contact panel so that it may be used with a different refrigerant.
It is also apparent that the adjustable contact panel system has practical limitations with respect to providing for selecting incremental changes in the rate of refrigerant to be dispensed. For example, the contact panel system used on a six pound refrigerant dispensing system, provides for selecting or changing the weight of liquid refrigerant dispensed in increments of 1/2 ounce. However, it has occasionally been found desirable to provide for selecting the weight of liquid to be dispensed in increments of less than 1/2 ounce. It is also difficult to produce a sliding contact panel assembly which corresponds to the metric system and provides for dispensing liquid refrigerant in grams and kilograms.